I called the local BMW dealer and asked this same question..
the response I got was..
"we use Autoglym Silver wheel restorer..."
Can't vouch for it myself...
I've used sperex/symoniz VHT paint, it' brighter than the std Silver..
be interesting if someone could measure the heat the engine produces when at temperature.. around cylinder heads especially.. the smooth hammerite Silver is Very close to the original colour. but I dont know about the temperatures or resistance to petrol etc..
Hammerite is an enamel and as such is pretty heat resistant, it just won't stand exhaust manifold levels of heat (years ago I painted a car silencer with it and it discoloured but lasted well).
I've done a two-valve boxer engine in silver, it's fine on the heads.
(Look closely at any four-valve boxer head, close to the exhaust manifold studs and you will see that the original paint discolours slightly too).
Just a note, the Satin version is a better match for black engines but it takes a lot longer to harden than the high gloss, an air-gun helps it to cure quicker.
Remember too that once you use enamel paints, although it will take over non-enamels, it won't allow non-enamels to take without some serious crinkling. You need to remove the enamel first.